Bright terrain on Ganymede is pervasively deformed by crosscutting
sets of subparallel grooves. High resolution images of bright
terrain returned by the Galileo spacecraft, including recent G7
and G8 observations, have greatly improved our understanding of
the morphology of grooves and their relationships to each other
and their surroundings. The grooves in the Galileo images have
been interpreted as predominantly extensional tectonic structures.
By examining the crosscutting relationships of these structures in
high resolution Galileo images, a sequence of events is derived
for several small target areas. The principles and observations
gained from these small areas are then applied to the surrounding
regions of grooved terrain imaged by Voyager at lower resolution.
Application of this analysis to a region in the area of Uruk
Sulcus and Nippur Sulcus on the antijovian hemisphere leads us to
two preliminary conclusions about the nature of grooved terrain on
Ganymede: (1) The style of extensional deformation which formed
the grooves is distinct in each stratigraphic interval. In the
area mapped so far, early structures are interpreted as horsts and
graben, and later structures are interpreted as indicative of tilt
block normal faulting and necking of the brittle lithosphere. (2)
The orientation of grooves in a given stratigraphic interval is
consistent over large distances and changes between different
intervals, indicating that the direction of extensional stress has
changed through time. In the mapped region, the earliest
extensional stresses are oriented in a W-E or NW-SE direction, and
the most recent stresses rotated to a NE-SW direction. These
observations do not favor low-strain or highly localized
mechanisms for the formation of grooved terrain such as the
cooling of cryovolcanic flows or the surface expression of small
convection cells, and instead favor large-scale mechanisms such as
tidal forces and/or global volume change from differentiation.